1. Field of the Invention
This invention relates to a flow meter, and more particularly to a magnetic flow meter composed of a sensor for producing a voltage signal proportional to the flow rate of liquid flowing through the measuring tube and a converter for converting the output signal of the sensor into the measured value of flow rate, with the converter being installed sufficiently away from the sensor.
2. Description of the Related Art
Widely known magnetic flow meters are generally made up of a sensor and a converter with a signal cable connecting them with each other. The sensor is provided with a measuring tube through which a fluid to be measured flows and a exciting coil that generates a magnetic field across the measuring tube. The measuring tube is provided with a pair of electrodes facing each other across the tube.
With a liquid flowing through the measuring tube, the sensor produces a differential voltage across a pair of the electrodes proportional to the flow rate, when an electric field generated by the exciting coil is applied to the measuring tube.
The converter is provided with an amplifier to which the differential voltage across the electrodes of the sensor is applied via a signal cable. It obtains a flow rate by processing the differential voltage amplified at the amplifier. It is also provided with an exciting circuit that supplies exciting current to the exciting coil of the sensor.
The sensor is separated from the converter to prevent the sensor from being installed in a poor maintenance environment such as inside the pit. It is desirable that the sensor should be placed where maintenance can be easily done.
For example, when the sensor is installed in an area of an explosion-proof atmosphere, the converter is placed in a safe area. In this case, it is difficult to secure a safe area of, for example, the order of some 10 m.sup.2 meters to install the converter in an explosion-proof atmosphere. This makes the separating distance between the sensor and the converter as long as from 300 to 1000 m or more.
The cable for transmitting a voltage signal to a remote place is made up of conducting wires wrapped with an insulating film and then covered with a shielding tube. Because a floating capacitance and an insulating resistance exit between the conducting wires and shielding tube, as the signal cable to transmit the differential voltage across the electrodes of the sensor to the converter becomes longer, the floating capacitance of the cable increases, which makes the cable impudance seen from the electrodes smaller. The longer cable also decreases the insulating resistance of the signal cable. Therefore, the length of the cable is limited by the capacitance and resistance of the cable.
For example, with a fluid conductivity of 5 .mu./cm, the length of the cable is limited to approximately 30 m, which is not long enough to separate the sensor from the converter.